Change a {MaybeHandle} to {Handle}. We do utf8-validation during decode
time (and verify this via DCHECK in {ExtractUtf8StringFromModuleBytes}),
so the only case where it could happen that we return an empty handle
there would be in an out-of-memory situation, and this is not handled
correctly anyway, so it is better to just crash in that case (via
{ToHandleChecked}).

R=ahaas@chromium.org

Bug: chromium:1036737
Change-Id: I6a0d94d920ab5dd7deecfa3e3033bdb5d266ffa7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1993288Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65679}